The present invention relates generally to radio frequency signal receivers, and more particularly to mixers and related circuits for use in radio frequency receivers.
Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, and the like. RF receivers all require frequency translation or mixing. For example, a television receiver may translate one channel in the band of 48 MHz to 870 MHz to an intermediate frequency of 44 MHz.
The majority of today""s RF receivers perform the necessary frequency translation or mixing using an oscillator and analog multiplier. FIG. 1 illustrates in schematic form a local oscillator and mixer circuit 60 known in the prior art that uses this technique. Circuit 60 includes an oscillator 62 and a mixer 64. Oscillator 62 forms local oscillator signal labeled xe2x80x9cLOxe2x80x9d in the form of a sine wave having a frequency of fLO. Mixer 64 mixes the RF input signal having desired spectral content at fIN with the local oscillator signal to form an output voltage signal labeled xe2x80x9cVOUTxe2x80x9d having spectral content at frequencies equal to the sum and difference of the input frequencies, namely fIN+fLO and fINxe2x88x92fLO.
Mathematically, the RF input signal can be expressed as:
RF=AIN(COS xcfx89INt)xe2x80x83xe2x80x83[1]
where AIN is the amplitude of the RF input signal, and xcfx89IN is the radian frequency. Similarly the local oscillator signal can be expressed as:
LO=ALO(cos xcfx89LOt)xe2x80x83xe2x80x83[2]
where ALO is the amplitude of the local oscillator signal, and xcfx89LO is the radian frequency. The product formed at the output of the mixer can be expressed as:                                                                         V                OUT                            =                            ⁢                                                [                                                            A                      IN                                        ⁡                                          (                                              cos                        ⁢                                                  xe2x80x83                                                ⁢                                                  ω                          IN                                                ⁢                        t                                            )                                                        )                                ⁡                                  [                                                            A                      LO                                        ⁡                                          (                                              cos                        ⁢                                                  xe2x80x83                                                ⁢                                                  ω                          LO                                                ⁢                        t                                            )                                                        ]                                                                                                        =                            ⁢                                                                                          A                      IN                                        ⁢                                          A                      LO                                                        2                                ⁡                                  [                                                                                    cos                        ⁡                                                  (                                                                                    ω                              IN                                                        -                                                          ω                              LO                                                                                )                                                                    ⁢                      t                                        +                                                                  cos                        ⁡                                                  (                                                                                    ω                              IN                                                        +                                                          ω                              LO                                                                                )                                                                    ⁢                      t                                                        ]                                                                                        [        3        ]            
One of these components forms the channel spectrum translated to the desired frequency and the other component can be filtered out. Oscillator 62 can be implemented by a tuned inductor-capacitor (LC) oscillator, a charge relaxation oscillator, or a ring oscillator.
An alternative is shown in FIG. 2, which illustrates in partial block diagram and partial schematic form another local oscillator and mixer circuit 80 known in the prior art. In circuit 80 a digitally synthesized oscillator 82, also known as a direct digital frequency synthesizer (DDFS), is used to generate the LO signal. The output of DDFS 82 is converted into an analog signal using a digital-to-analog converter (DAC) 84 for input to a mixer 86. This technique of generating the local oscillator signal has several advantages compared to the analog oscillator used in FIG. 1, including wide tuning range, high noise immunity, minimal self-mixing, and minimal leakage. However the Nyquist criterion that forces DDFS 82 to be clocked at greater than twice the highest oscillation frequency has so far limited its use to low-frequency applications.
It would be desirable to have circuitry for use in an RF receiver that is suitable for higher frequency applications. Such circuitry and related methods are provided by the present invention, whose features and characteristics will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
Accordingly the present invention provides, in one form, a circuit adapted for use in a frequency receiver. The circuit includes a transconductance amplifier, a direct digital frequency synthesizer, and a digital-to-analog converter (DAC). The transconductance amplifier has an input terminal for receiving a radio frequency signal, and an output terminal for providing a current signal. The direct digital frequency synthesizer has an output terminal for providing a digital local oscillator signal at a selected frequency. The DAC has a first input terminal coupled to the output terminal of the transconductance amplifier, a second input terminal coupled to the output terminal of the direct digital frequency synthesizer, and an output terminal for providing an output signal.
In another form, the present invention provides a method for use in a radio frequency receiver. A radio frequency signal is received and converted into a current signal. A digital local oscillator signal is generated and is mixed with the current signal to form an analog output signal.